1. Field of the Invention
The present invention relates to an exposure apparatus which is used in the photolithography process for manufacturing microdevices, such as semiconductor elements, or liquid crystal elements, image pick-up elements (CCD), thin film magnetic heads, opto-magnetic discs, etc., and in particular, to a scanning exposure apparatus of the so-called slit scan method, the step and scan method, or the like, which scans a mask and a photosensitive substrate in synchronization so as to perform exposure of said substrate with a pattern image of said mask.
2. Related Background Art
In producing semiconductor elements or liquid crystal display elements by the photolithography process, a projection exposure apparatus is used for projecting an image of a pattern on a photomask or a reticle (the both will be hereinafter referred to as xe2x80x9creticlexe2x80x9d) through a projection optical system onto a substrate (a wafer, a glass plate, or the like) on which a coating of photoresist or the like is given, and thereby effecting exposure of the image on the substrate. Such a projection exposure apparatus is required to perform such accurate overlay exposure that a presently exposed reticle pattern is accurately overlaid across the entire exposure field over a chip pattern on the substrate such as a wafer, which was formed by previous exposure and process treatments. Namely, it is necessary that the exposure is conducted while keeping high overlay accuracy between the pattern formed on the substrate and the pattern of the reticle.
A semiconductor element or the like is normally constructed in lamination of many layers of circuit patterns overlaid on a substrate. For example, if the circuit patterns in the layers are formed using different projection exposure apparatus and if there is a difference exceeding a predetermined acceptable value between a magnification error of a projection exposure apparatus which performed previous exposure of a circuit pattern in a previous layer and a magnification error of a projection exposure apparatus which is to perform present exposure of a circuit pattern in a next layer, the overlay accuracy is degraded so as to lower the yield of semiconductor elements. Also, an overlay error occurs if there is a difference in distortion of projected image of mask pattern between two projection exposure apparatus. There is also a case in which a substrate is distorted by heating due to various process treatments after exposure, which results in distorting a previously exposed pattern. The distortion of pattern in this case will eventually be similar to the distortion of projected image caused by a previously used projection exposure apparatus.
Concerning this, conventional projection exposure apparatus were generally apparatus (steppers) of the full exposure method (or xe2x80x9cfull field methodxe2x80x9d) which projected a reduced image of reticle pattern over the entire exposure field on a photosensitive substrate by one operation. In applications with such projection exposure apparatus of the full exposure method, imaging properties of a projection optical system are actively adjusted by driving some lens elements in the projection optical system or a reticle along the optical axis or inclining it with respect to the optical axis for example to change a projection magnification of projected pattern or to distort the projected pattern in the form of trapezoid or barrel. As described, there are suggestions for the method in which exposure is effected while the distortion of projected image of presently exposed pattern is kept correspondent to that of previously exposed pattern (see U.S. Pat. Nos. 4,734,746 and 5,117,255).
The suggestions for improvement in overlay accuracy as described above were made under a premise of use of the full exposure method (full field method). However, a recent trend is to increase the size of a chip pattern of semiconductor element, which requires the projection exposure apparatus to increase an exposure area, for permitting a pattern with a larger area on the reticle to be exposed on the photosensitive substrate. To handle the larger area exposure of transferred pattern and the limit of exposure field of projection optical system, there are suggested projection exposure apparatus of the so-called slit scan exposure method, which moves the reticle and the photosensitive substrate in synchronization relative to a rectangular, arcuate or hexagonal illumination area (hereinafter referred to as xe2x80x9cslit illumination areaxe2x80x9d), for example, whereby patterns with a larger area than the slit illumination area on the reticle are successively projected and exposed on the substrate.
The scanning exposure method has such advantages that illumination uniformity on the reticle (or a wafer) is improved, a distortion, a curvature of the image field, astigmatism, and the like of the projection optical system are reduced, and the uniformity of focusing position in the exposure field is improved, because the illumination area on the reticle in the slit scan exposure method is smaller than that in the full exposure method. Also, the slit scan exposure method has another advantage that large area exposure is possible in the scanning directions of the reticle and the substrate without being affected by the limit of field size of projection optical system.
The conventional projection exposure apparatus of the slit scan exposure method as described above, however, had such a disadvantage that if the magnification error was simply corrected only by the projection optical system, the magnification error could be corrected in the non-scanning direction perpendicular to the scanning direction, but the magnification error could not be fully corrected in the scanning direction.
In addition, since the conventional projection exposure apparatus of the slit scan exposure method have the width of slit illumination area in the scanning direction different from that in the non-scanning direction, they are likely to have a difference between the magnification error in the scanning direction and the magnification error in the non-scanning direction in each shot area because of a bias of heat distribution caused by absorption of exposure light in the projection optical system and because of a difference between a shot size in the scanning direction and a shot size in the non-scanning direction on the substrate. Accordingly, it is to be desired that the apparatus are arranged to correct the magnification error in the scanning direction and the magnification error in the non-scanning direction independently of each other in particular.
Further, since the conventional slit scan exposure method uses only a part of field of the projection optical system and a same pattern passes through a plurality of portions in the field of projection optical system upon scanning exposure, it cannot be possible to distort the projected image as a whole in the trapezoidal shape or in the barrel shape by simply inclining a lens element in the projection optical system.
The method for improving the overlay accuracy in the slit scan exposure method as described above is disclosed for example in Japanese Patent Publication No. 5-29129. The projection optical system in the exposure apparatus as disclosed in the publication is of the reflection type, using only mirrors such as a concave mirror and a convex mirror. Also, the mask and the substrate are held on a single scanning member (scanning frame) so that they are moved together in a same direction.
An amount of positional deviation between the mask and the substrate is measured at a plurality of portions during movement of the mask and the substrate. A fine feed mechanism for adjusting the position of wafer by a fine amount is driven based on the detection result to change a relative position between the mask and the substrate.
The above conventional technology, however, simply adjusts the position of wafer by a fine amount and has a disadvantage that, for example, if a shot area on the substrate is distorted in a trapezoidal shape or in a barrel shape, the pattern of mask cannot be overlaid with accuracy as a whole on a shot area.
Further, the scanning exposure method has such a disadvantage that the image quality may be deteriorated by a scanning exposure, depending on a distortion of a projected image due to an aberration of the projection optical system. This deterioration in the image quality will be specifically described with reference to FIGS. 15A to 15C. In FIGS. 15A to 15C, a scanning direction of the photosensitive substrate is represented by +x direction (or xe2x88x92x direction), while a non-scanning direction perpendicular to the x direction is y direction. When a projection optical system has no aberration, an exposure field of said projection optical system (i.e., a projected image with no distortion) is a rectangular exposure area 141 having the width of D in the x direction.
When the rectangular exposure area 141 is deformed into an exposure area 142A in the form of a parallelogram which is inclined by the angle xcex3[rad] with respect to the axis x due to an aberration of the projection optical system, as shown in FIG. 15A, if the photosensitive substrate is scanned in the x direction, the projected image is moved to the non-scanning direction (y direction) on the photosensitive substrate only by Dxc2x7xcex3 or around while one point on the photosensitive substrate intersects the exposure area 142A, thereby deteriorating the quality of the image. Also, when the projection magnification with respect to the scanning direction (x direction) of the projection optical system is shifted by xcex2, the exposure area 141 having the width D is deformed into an exposure area 142B having the width (1+xcex2) D, as shown in FIG. 15B. In this case, since the projected image is moved on the photosensitive substrate in the scanning direction in excess by xcex2xc2x7D while the one point on the photosensitive substrate intersects the area having the width D within the exposure area 142B along the x direction, the image quality is also deteriorated.
Further, when the projection magnification of the projection system varies in the non-scanning direction and the sides of the exposure area 141 extending in the scanning direction are symmetrically inclined by the angle xcex4[rad], respectively, so that said exposure area 141 is deformed into an exposure area 142C in the form of trapezoid, as shown in FIG. 15C, the projected image is moved on the photosensitive substrate in the non-scanning direction by Dxc2x7xcex4 at the maximum while one point on the photosensitive substrate intersects the exposure area 142C along the x direction, which also results in image deterioration.
Here, a pattern on the reticle is a line-and-space pattern which is arranged periodically in the scanning direction, the pitch of the projected image of said pattern on the photosensitive substrate in the x direction is denoted by p, and this pitch p is about two times as much as the exposure wave length. In this case, the light intensity distribution I(x) of said projected image can be expressed by Icos{(2xcfx80/p)xc2x7x}. If, as an extreme example, only an optical image is synthesized by shifting a phase of a projected image thereof by xcex94/2, the light intensity distribution of said optical image IT(x) is expressed as follows:                                           IT            ⁡                          (              x              )                                =                      xe2x80x83                    ⁢                                    (                              1                /                2                            )                        [                                          I                ⁢                                  xe2x80x83                                ⁢                cos                ⁢                                  {                                                                                    (                                                  2                          ⁢                                                      π                            /                            p                                                                          )                                            ⁢                      x                                        -                                          Δ                      /                      2                                                        }                                            +                                                                                xe2x80x83                    ⁢                      I            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          {                                                                    (                                          2                      ⁢                                              π                        /                        p                                                              )                                    ⁢                  x                                +                                  Δ                  /                  2                                            }                                ]                                              =                      xe2x80x83                    ⁢                      I            ⁢                          xe2x80x83                        ⁢                          cos              ⁡                              (                                  Δ                  /                  2                                )                                      ⁢            cos            ⁢                          {                                                (                                      2                    ⁢                                          π                      /                      p                                                        )                                ·                x                            }                                            "AutoLeftMatch"
As clearly seen from the above equation, a contrast of said synthetic image is decreased to cos(xcex94/2) times as much a contrast as the original projected image. Thus, a degree of resolution of the synthetic image is lowered, which results in deterioration of the image quality.
The distortion of the projected image shown in FIGS. 15A to 15C is an anisotropic distortion which is differently distorted for each direction. On the contrary, when the distortion of said projected image is isotropic, that is, when only the projection magnification varies, the deterioration in the image quality caused by a change in the magnification can be easily coped with by, for example, regulating a ratio between a scanning speed of the reticle and that of the photosensitive substrate.
It is an object of the present invention to provide a scanning exposure method and apparatus capable of overlaying exposure with high accuracy.
It is another object of the present invention to provide a scanning exposure method and apparatus which can independently correct a magnification error between a pattern image of the mask and a processing area on the photosensitive substrate in a scanning direction and the non-scanning direction.
It is still another object of the present invention to provide a scanning exposure method and apparatus which can form the pattern image of the mask in a processing area on the photosensitive substrate without deteriorating the image quality even if said processing has been deformed.
It is still another object of the present invention to provide a scanning exposure method and apparatus which does not deteriorate the quality of an image exposed on the photosensitive substrate (degree of resolution) even when a projected image by the projection optical system is anisotropically deformed.
It is still another object of the present invention to provide a method of manufacturing microdevices with high accuracy by performing the scanning exposure of the photosensitive substrate with the pattern image of the mask.
A scanning projection exposure apparatus according to the present invention, for example as shown in FIG. 1, is an apparatus provided with an illumination optical system (1-10) for irradiating illumination light from a light source onto a predetermined illumination area on a mask (R) and a projection optical system (PL) for projecting a pattern formed on said mask (R) onto a photosensitive substrate (W), which synchronously moves said mask (R) and said substrate (W) in predetermined scanning directions perpendicular to an optical axis (AX) of said projection optical system (PL) thereby to effect scanning exposure of a projected image of said pattern on said substrate (W), comprising:
means (16) for inputting information on a distortion of a pattern having been already formed on said substrate (W), in a direction perpendicular to said scanning directions; and
means (35) for controlling imaging properties of said projection optical system, based on said information thus input.
The apparatus of the present invention preliminarily obtains a state of a distortion for example of a chip pattern having been already formed on the substrate (W), in the direction perpendicular to the scanning directions. It can be obtained for example by detecting alignment marks provided around the chip pattern by means of distortion detecting means (30, 31). Or, characteristics of a distortion of projected image by an exposure apparatus for previous exposure may be input through input means (22). Then, in accordance with the state of distortion of the pattern on the substrate in the direction perpendicular to the scanning directions, the imaging properties control means (34, 35, 36) drives a lens element in the projection optical system (PL) or controls the pressure or the temperature of gas in a hermetically sealed space between some lenses, for example. This permits the projected image of the pattern on the mask to be overlaid with high accuracy on the pattern on the substrate in the direction perpendicular to the scanning directions.
Another scanning projection exposure apparatus according to the present invention, for example as shown in FIG. 1, is an apparatus provided with an illumination optical system (1-10) for irradiating illumination light from a light source onto a predetermined illumination area on a mask (R) and a projection optical system (PL) for projecting a pattern formed on said mask (R) onto a photosensitive substrate (W), which synchronously moves said mask (R) and said substrate (R) in predetermined scanning directions perpendicular to an optical axis (AX) of said projection optical system (PL) thereby to effect scanning exposure of a projected image of said pattern on said substrate (W), comprising:
input means (22) for inputting information on a distortion of a pattern having been already formed on said substrate (W);
first means (34, 35, 36) for controlling imaging properties of said projection optical system (PL) in accordance with said information thus input; and
second means (16, 17, 25) for controlling a relative scanning speed between said mask (R) and said substrate (W) in accordance with said information thus input;
wherein in said scanning exposure, a projection magnification of said projected image in said scanning directions is adjusted through said second means (16, 17, 25) and a projection magnification of said projected image is isotropically or anisotropically adjusted through said first means (34, 35, 36), whereby said projected image is overlaid on said pattern having been already formed on said substrate (W).
A further scanning projection exposure apparatus of the present invention is an apparatus provided with an illumination optical system (1-10) for irradiating illumination light from a light source onto a predetermined illumination area on a mask (R) and a projection optical system (PL) for projecting a pattern formed on said mask (R) onto a photosensitive substrate (W), which synchronously moves said mask (R) and said substrate (W) in predetermined scanning directions perpendicular to an optical axis (AX) of said projection optical system (PL) thereby to effect scanning exposure of a projected image of said pattern on said substrate (W), comprising:
distortion detecting means (30, 31) for detecting a state of a distortion of a pattern having been already formed on said substrate (W);
first means (34, 35, 36) for controlling imaging properties of said projection optical system (PL) based on a detection result of said distortion detecting means (30, 31); and
second means (16, 17, 25) for controlling a relative scanning speed between said mask (R) and said substrate (W) based on the detection result of said distortion detecting means (30, 31);
wherein in said scanning exposure, a projection magnification of said projected image in said scanning directions is adjusted through said second means (16, 17, 25) and a projection magnification of said projected image is isotropically or anisotropically adjusted through said first means (34, 35, 36), whereby said projected image is overlaid on said pattern having been already formed on said substrate (W).
The apparatus of the present invention preliminarily obtains a state of a distortion of a chip pattern having been already formed on the substrate (W), for example. This can be done by detecting the state of the distortion of the pattern on the substrate by the distortion detecting means (30, 31) or by inputting characteristics of the distortion of projected image by an exposure apparatus for previous exposure through the input means (22). Then, in accordance with the state of distortion of the pattern on the substrate, the first means (34, 35, 36) drives a lens element in the projection optical system (PL) or controls the pressure or the temperature of gas in a hermetically sealed space between some lenses, for example. This can adjust a projection magnification of the projected image of the pattern on the mask in the direction perpendicular to the scanning directions. Also, the second means (16, 17, 25) adjusts a relative speed between the mask (R) and the substrate (W) in accordance with the state of distortion of the pattern on the substrate. This can adjust a projection magnification of the projected image of the pattern on the mask in the scanning directions.
For example, consider a case in which only a magnification component of the pattern on the substrate is changed as shown in FIG. 6. Letting xcex20 be a projection magnification of the projection optical system and V be a scanning speed of the mask in its scanning direction (referred to as xe2x80x9cxe2x88x92X directionxe2x80x9d), a scanning speed of the substrate in the X direction is defined as xcex20V. If a magnification error of the pattern on the substrate in the scanning direction is a and a magnification error in the direction perpendicular to the scanning direction is xcex3, the main control system (16) sets the projection magnification of projection optical system to (xcex20+xcex3) and the scanning speed of the substrate upon scanning exposure to (xcex20+xcex1)V. By this, the projected image of the pattern on the mask can be accurately overlaid on the pattern on the substrate.
Also, for example in case a pattern on the substrate is distorted in a trapezoidal shape as shown in FIG. 8 or in FIG. 9, the projected image of the pattern on the mask can be overlaid with high accuracy on the pattern on the substrate in accordance therewith by controlling the imaging properties of projection optical system (PL) so as to distort the projected image of the pattern on the mask and continuously changing the imaging properties of the projection optical system and the scanning speed of the substrate during scanning exposure.
A scanning exposure method according to the present invention is arranged, with respect to a projection optical system for projecting a pattern image on the mask to the photosensitive substrate, to scan the mask in a predetermined direction intersecting the optical axis of said projection optical system while scanning the substrate in a direction corresponding to said predetermined direction in synchronization therewith, thereby transferring the pattern image on said mask onto said substrate. Then, a distortion is given to the pattern image which is transferred from the mask onto the substrate during the scanning exposure in accordance with anisotropical component values (such as degree of parallelogram, degree of rectangle, degree of trapezoid, etc.) within the distortion of the projected image by the projection optical system. Further, in this case, it is preferable that the mask pattern should be distorted in advance so as to set off the anisotropic component values in the distortion of said projected image.
According to such scanning exposure method of the present invention, a pattern image of the mask is distorted by adjusting an angle between the scanning direction of the mask and that of the substrate, a relative scanning speed, a projection magnification, or the like, at the scanning exposure in accordance with the anisotropic component values within the distortion of the projected image by the projection optical system. For this reason, if there is an anisotropic distortion of the projected image, deterioration in the image quality of a pattern image which is finally formed on the substrate can be suppressed to the minimum. Therefore, the resolution of the image finally formed on the substrate is not lowered. Further, in the distortion of the projected image, the anisotropic components which can be adjusted and controlled at the scanning exposure are not necessarily adjusted exactly to predetermined allowable values at the time of assembling and regulating of the projection optical system, but may be roughly adjusted, which brings about an advantage that the manufacturing time of the projection optical system and, in its turn, that of the exposure apparatus can be reduced.
If there is no distortion in a pattern on the mask, an anisotropic distortion remains in an image which is formed on the substrate in accordance with a distortion of a projected image by the projection optical system. Thus, in order to remove this remaining distortion, the pattern on the mask is distorted in such a manner that it should set off the distortion of the projected image in advance. In other words, it is possible to obtain a pattern image without distortion after the scanning exposure by reversely correcting the pattern on the mask for the distortion of the projected image, thereby improving the overlay accuracy between adjacent layers.
A scanning exposure apparatus of the present invention is provided with a projection optical system for projecting a pattern image on a mask onto a photosensitive substrate, a mask stage for scanning the mask in a predetermined direction crossing the optical axis of said projection optical system, and a substrate stage for scanning the substrate in a direction crossing the optical axis of the projection optical system and corresponding to said predetermined direction, wherein the pattern image on the mask is transferred onto the substrate by synchronously scanning the mask and the substrate with respect to the projection optical system. The apparatus is further provided with a memory for storing anisotropic component values within a distortion of a projected image by the projection optical system, and a distortion control device for distorting the pattern image which is transferred onto the substrate at the scanning exposure in compliance with the anisotropic component values of the image distortion stored in said memory.
The distortion control device is, for example, an angle controller for varying a relative angle between a scanning direction of the mask by the mask stage and a scanning direction of the substrate by the substrate stage, or a speed controller for adjusting a relative speed between a scanning speed of the mask by the mask stage and a scanning speed of the substrate by the substrate stage, or a magnification controller for adjusting a projection magnification of the projection optical system. Further, the distortion control device may be constituted by combining any two out of the angle controller, the speed controller, and the magnification controller, or by combining all (three) of the controllers. It is preferable that an exposure amount distribution control device should be also provided for controlling distribution of an exposure amount on the substrate in accordance with an adjusted amount of the pattern image on the mask by the distortion control device.
According to the scanning exposure apparatus of the present invention, even if there is an anisotropic distortion in the projected image, deterioration in the quality of an image which is formed on the substrate can be finally suppressed to the minimum. Thus, the resolution of the pattern image is not damaged. Further, since the anisotropic components are not necessarily adjusted exactly to predetermined allowable values at the time of assembling and regulating of the projection optical system, but may be roughly adjusted, the manufacturing time of the projection optical system (exposure apparatus) can be reduced. By distorting the mask pattern in such a manner that a distortion of the projected image should be set off in advance, it is possible to obtain a pattern image without distortion after the scanning exposure, thereby improving the overlay accuracy between adjacent layers.
If the distortion control device is an angle controller which changes a relative angle between the scanning direction of the mask by the mask stage and the scanning direction of the substrate by the substrate stage, a distortion, for example, in the form of a parallelogram can be corrected. If the distortion control device is a speed controller which adjusts a relative speed between the scanning speed of the mask by the mask stage and the scanning speed of the substrate by the substrate stage, a distortion, for example, in a rectangular shape can be corrected. If the distortion control device is a magnification controller which adjusts a projection magnification of the projection optical system, a distortion, for example, in a trapezoidal shape which is symmetrical with respect to the axis parallel to the scanning direction can be corrected. Further, a trapezoidal distortion which is symmetrical in the non-scanning direction can be also corrected by combining the angle controller and the magnification controller and by adjusting a relative angle and a projection magnification during the scanning exposure.
When there is provided an exposure amount distribution control device for controlling an amount of exposure on the substrate or the distribution thereof in accordance with an amount of adjustment of a pattern image on the mask by the distortion control device, even if a distortion of the image on the substrate is corrected by the distortion control device, no unevenness occurs in the amount of exposure on the substrate, or an amount of exposure exceeding or less than a predetermined range is not given, that is, an amount of exposure on the entire region on which a pattern image can be formed on the substrate can be made substantially uniform and the amount of exposure can be maintained at a proper value.